weird issue with index macro

using KernelAbstractions

@kernel function kernel_index!(a)
  i = KernelAbstractions.@index(Global)
  @inbounds a[i] = i
end

let
  a = zeros(1)
  kernel! = kernel_index!(CPU(), 1, 1)
  event = kernel!(a)
  wait(event)
end

fails with

TaskFailedException:
MethodError: no method matching __index_Global_Linear()
Stacktrace:
 [1] call at /home/mwarusz/.julia/packages/Cassette/7OymZ/src/context.jl:450 [inlined]
 [2] fallback at /home/mwarusz/.julia/packages/Cassette/7OymZ/src/context.jl:448 [inlined]
 [3] _overdub_fallback at /home/mwarusz/.julia/packages/Cassette/7OymZ/src/overdub.jl:486 [inlined]
 [4] ##cpu_kernel_index!#423 at /home/mwarusz/repos/KernelAbstractions.jl/src/macros.jl:4 [inlined]
 [5] (::KernelAbstractions.var"#22#27"{Cassette.Context{nametype(CPUCtx),KernelAbstractions.CompilerMetadata{KernelAbstractions.StaticSize{(1,)},KernelAbstractions.StaticSize{(1,)},false,Int64,Nothing,Nothing},Nothing,KernelAbstractions.var"##PassType#422",Nothing,Cassette.DisableHooks},KernelAbstractions.Kernel{CPU,KernelAbstractions.StaticSize{(1,)},KernelAbstractions.StaticSize{(1,)},var"###cpu_kernel_index!#423"},Tuple{Array{Float64,1}}})() at ./threadingconstructs.jl:113
Stacktrace:
 [1] macro expansion at ./task.jl:319 [inlined]
 [2] macro expansion at /home/mwarusz/repos/KernelAbstractions.jl/src/backends/cpu.jl:38 [inlined]
 [3] (::KernelAbstractions.var"#18#23"{KernelAbstractions.Kernel{CPU,KernelAbstractions.StaticSize{(1,)},KernelAbstractions.StaticSize{(1,)},var"###cpu_kernel_index!#423"},Tuple{Array{Float64,1}},Int64,Bool})() at ./threadingconstructs.jl:113
Stacktrace:
 [1] wait at ./task.jl:251 [inlined]
 [2] wait at /home/mwarusz/repos/KernelAbstractions.jl/src/backends/cpu.jl:7 [inlined]
 [3] wait(::KernelAbstractions.CPUEvent) at /home/mwarusz/repos/KernelAbstractions.jl/src/backends/cpu.jl:6
 [4] top-level scope at /home/mwarusz/repos/KernelAbstractions.jl/examples/unroll_issue.jl:13
in expression starting at /home/mwarusz/repos/KernelAbstractions.jl/examples/unroll_issue.jl:9

Reproduce the failure from examples/unroll_issue.jl, then inspect src/macros.jl and src/backends/cpu.jl, which appear in the stack trace. Trace how @index(Global) is expanded and invoked in the CPU path, and verify that the example completes without the __index_Global_Linear() MethodError.